1. Field of the Invention
The present invention relates generally to a telephony device and, more particularly, to a telephony device that uses powers from two or more sources.
2. Background of the Invention
Conventional telephones includes those that are line (PSTN) powered. Line powered telephones usually offer only very basic functionality as there are very stringent limits imposed by regulatory authorities on the amount of current that can be drawn off the PSTN line both for on-hook and off-hook conditions. For example, during an on-hook condition, the telephone must represent an impedance of greater than 5 MΩ (5,000,000 Ohms). With on-hook voltages as low as 25 V (or possibly lower than 25 V depending on the central office, loop length, and other factors), this represents a maximum current draw of less than 5 μA (5 micro amps). A typical radio transceiver device, e.g., a cordless telephone, operating in standby mode would have a peak current draw many time greater than this, thereby precluding the possibility of any type of radio communication while the device is on-hook. This is clearly an unacceptable functional limitation as any cordless telephony device must be able to respond to radio communication initiated by the cordless handset prior to the base device going off-hook.
Most cordless telephones today have advanced functions that require the base unit to be connected to a PSTN line and an external power source to enable proper operation of the advanced functions. The need for the external power source, e.g., AC power outlet, is associated with a number of disadvantages. First, a power adapter that is cumbersome and esthetically unappealing is required. Second, the telephone must be placed at a location where the PSTN line and the AC power outlet are co-located. Third, the telephone ceases to operate during power failure events.
Therefore, users have to choose between a line-powered corded telephone that offers very simple functionality with no external power required and a more fully featured device that utilizes an external power connection. There is not any telephony device in the market that provides full functionality without requiring an external power connection.